Along with an increase in integration of semiconductor devices, a wiring technology is increasingly proceeding in the direction of miniaturization and an increase in number of layers. Then, due to the increase in number of layers in the wiring technology, the difference in level of a semiconductor substrate surface increases, and the gradient of wiring with respect to the substrate becomes steep. Therefore, the processing accuracy and reliability of the wiring to be formed in an upper portion of the substrate tend to decrease.
In order to solve the above-mentioned problem, a chemical mechanical polishing method (hereinafter sometimes referred to as “CMP method”) is drawing attention. The CMP method is a polishing method involving a combination of a mechanical removal action of particles, including abrasive grains, and a chemical removal action of a processing liquid, and in the CMP method, an abrasive (aqueous dispersion) in which fine particles, such as silica or alumina, are mixed and dispersed in an alkaline or acidic chemical aqueous solution and a polishing pad are used. For example, in a silicon wafer surface, it is generally considered that a hydrate film generated on the wafer surface, rather than the wafer surface itself, is removed with the abrasive grains, and thus processing proceeds. The CMP method has a feature in that the depth of an affected layer is substantially negligible, and the wafer surface is mirror-finished with high efficiency.
An object is processed by selecting the polishing pad, abrasive grains, the kind, concentration, and pH of the processing liquid, and the like depending on the object to be processed. For example, in the CMP of an oxide film (SiO2), a hydrate such as Si(OH)4 formed through the cleavage of a Si—O—Si bond is removed with fine particles through use of a silica slurry having a pH controlled with an alkali, such as KOH. Meanwhile, the CMP of metal is considered to have a mechanism in which a metal oxide film generated on the surface of the metal is removed with fine particles through use of a slurry in which alumina or silica particles are dispersed in an acidic solution having an oxidant added thereto.
In recent years, such CMP method has rapidly spread as a procedure for flattening an interlayer insulating film such as a silicon oxide film, a metal film of aluminum, tungsten, copper, or the like forming a wiring layer, and a material such as TiN, TaN, or SiN during production steps of a semiconductor device. An abrasive to be used in the CMP method is required to have various performances such as low contamination with respect to an object to be polished, a small number of scratches, high polishing efficiency, and a high selection ratio of polishing a silicon oxide film.
The above-mentioned performances of the abrasive significantly depend on abrasive grain components, such as silica or cerium oxide, which serves as a main raw material. For example, an abrasive using fumed silica as abrasive grains, which has been often used hitherto, is excellent in purity and has high polishing efficiency. However, in the CMP method, this abrasive has a problem in that a large number of scratches are caused also by the influence of chemical components. In an abrasive using colloidal silica as abrasive grains, although scratches are reduced, this abrasive has polishing efficiency lower than that of fumed silica and also has a problem in terms of purity. It has been known that an abrasive using cerium oxide as abrasive grains has high polishing efficiency. However, a large number of scratches are caused by poor dispersion stability, and there is also a problem in terms of purity.
In Patent Literature 1, there is a disclosure of the comparison of scratch characteristics of slurries for chemical mechanical polishing using colloidal silica and fumed silica (hereinafter sometimes referred to as “slurry for CMP”), and there is also a disclosure of the tendency that a large number of scratches occur in the slurry for CMP using fumed silica compared to the slurry for CMP using colloidal silica.